That doesn't make any sense... 30 weight is the viscosity...
It does make sense and the industry standards allow them to produce a 30wt oil that can have different viscosities while still claiming, rightfully so, they are within the 30 wt spec.
According to the Society of Automotive Engineers (SAE) a 30 wt oil must have its viscosity fall within the following:
Not less than 9.3 cSt at 100C and not more than 12.5 cSt at 100C. That's a large spread however any oil that falls within the grade meets spec and can be used in a car specifying a 30 wt oil.
Taking it to real world oil numbers from the manufacturer's own sites, the Pennzoil Platinum 5w-30 I recommended has a 9.8 cSt value at 100C. That's barely within the grade and very close to a 20 wt oil which must have a value of less than 9.3.
On the thicker side, Castrol 0w-30 posts specs on their site showing a 12.3 cSt value. That's almost a 40 wt but still within the 30 wt requirement. The M1 0w-40 I'm currently running has a 12.9 cSt value which is nearly a 30 wt but if have any fuel dilution or shearing, it's solidly a 30 wt and within spec.
Thinner oils are supposed to help with mileage while affording protection and thicker helps with protection while taking a tick off of mileage. If you tow, run at hot ambient temps with a loaded car or run up grades the manual suggests a change to a thicker viscosity however as you can see, just changing to a thicker weight within grade will work.
For a 2.5 that requires a 20 wt oil, as you can see there is very little diffence between a 20 and 30 wt oils at 100C. Most 2.5s run a little hotter but 100C is the industry standard for measurement so a 20 wt is little thinner hence the thin 30 wt recomnendation.
This oil world is a crazy place but there's room, without running into warranty problems, to use a thinner or thicker oil within the requested spec.
EDIT for additional info:
To illustrate the differences at oil temp the industry uses 100C but from browsing this site numerous posts over the years have people reporting various oil temps on the 2.5 engines. A lot are less than 205F, some are higher at over 210, some spike even higher for brief periods and I'm using a guesstimate or WAG of 217F for illustration purposes. A search of posts shows a lot of different temps and 217 is not outside of the curve.
Using the Widman viscosity calculator one enters the 40C and 100C cSt values and then the temp for which you want to find the new value.
With the Pennzoil Platinum 5w-30 the respective values are 53.9 and 9.8 to start with.
217 converted is 103C and plugging that figure in, if your car runs at 217 the new cSt value is 9.2. That's a 20 wt oil viscosity. There may be other things in the oil that offer protection that are organic or not measurable by an ordinary UOA however if one focuses on a specific viscosity requirement, a thin 30 wt is very appropriate for the 2.5.